This invention relates to cardiac valve replacement in heart surgery, and particularly to the replacement of the mitral valve.
Cardiac valve replacement is a relatively common procedure. However, in comparison with aortic, tricuspid and pulmonary valve replacement procedures, mitral valve replacement procedures have exhibited the poorest results in terms of morbidity and mortality. Under normal conditions, the mitral valve is exposed to the greatest pressure and stress during the cardiac cycle, with pressures often exceeding 150 mm Hg.
The mitral valve is generally a thin continuous, flexible membrane, strengthened by collagen fibers, surrounding the left atrio-ventricular ring having two indentations or commissures dividing it into two principal trapezoidal leaflets of unequal size: an anterior or aortic leaflet and a posterior or mural leaflet. The membrane at the junction of the two leaflets has sufficient length to form two auxiliary cusps located at each commissure. The membrane is attached to over two-thirds of the circumference of the atrio-ventricular ring and to the base of the aorta just below the aortic valve. The free ends of the leaflets are attached to chordae tendineae at the ventricular surface and in the regions of the commissures. The other end of the chordae connect to the papillary muscles, with each papillary muscle receiving chordae from both leaflets.
During diastole, a normal mitral valve will have a measured circumference between about 8.5 to 11 cm (for adult males) and 7.5 to 10.5 cm (for adult females). The calculated circularized valve orifice diastolic diameter is between about 27 and 35 mm (for adult males) and 23.8 and 33.4 mm (for adult females) and the calculated diastolic cross-sectional area is between about 5.75 and 9.62 sq. cm. for adult males and 4.5 and 8.77 sq. cm. for adult females. In cases of congestive heart failure, these dimensions enlarge, with the circumference of the adult male valve orifice reaching as high as about 12 cm, or greater and the adult female valve orifice reaching as high as about 11 cm.
The dimensions of the anterior leaflet are between about 1.9 and 3.2 cm in length and 2.5 and 4.5 cm in width for adult males, and 1.8 to 2.7 cm in length and 2.4 to 4.2 cm in width for adult females. The posterior leaflet has dimensions of between about 1.0 to 2.5 cm in length and 2.5 to 4.1 cm in width for adult males, and 0.8 to 2.4 cm in length and 2.3 to 3.6 cm in width for adult females. The chordae tendineae for both adult males and females is between about 1.3 and 3.2 cm. As the apical zones of the cusps correspond, the body of the anterior cusp lies opposite the base of the shorter posterior cusp. The chordae tendineae of the posterior cusp are inserted into almost the entire undersurface of the cusp, whereas those of the anterior cusp are inserted into a zone along its periphery. The remaining larger central triangular portion of the anterior cusp is thinner and more mobile than the marginal zone since its components are not directly limited by the chordae tendineae.
During systole a large portion of the anterior cusp billows toward the left atrium above the level of the base of the posterior cusp with about thirty percent of the anterior cusp co-apting with about fifty percent of the posterior cusp. The anterior cusp swings upwards and backwards. The swing of the anterior cusp is made possible by three cooperative actions: the absolute length of the anterior cusp and its chordae tendineae, the relative increase in length caused by the systolic approximations of bases of the papillary muscles toward the mitral ring, and the stretching of the papillary muscles by the interventricular pressure acting on the under surfaces of the cusps. Systolic excursions of the cusps are possible well beyond the normal requirements for valve closure due to the length of the cusps and the chordae tendineae and the extendibility of the papillary muscles. Consequently, the mitral valve has a large closing reserve. During diastole, the atrioventricular ring dilates and the valve leaflets descend to rapidly open the valve. The specific gravity of the leaflets is close to that of the blood so that as the ventricular chamber fills, the leaflets begin to float upward toward the annulus, initiating closure of the mitral orifice.
There is normally an excess of cusp tissue in relation to the size of the mitral ring. For example, for a mitral valve orifice area of about 7.9 sq. cm., a leaflet area of about 13.9 sq. cm. is available for closure. Thus, immediate and complete closure of the mitral valve takes place during systole with the ventricular contraction narrowing the mitral ring by about twenty-six to thirty-five percent (in comparison to its diameter during diastole.) The decrease in size of the mitral ring exposes less of the mitral valve surface to the burden of left ventricle systolic pressure. Thus, the annulus changes size from a relatively large opening during diastole and a smaller opening during systole.
The ideal valve substitute should be designed to reproduce as accurately as possible the normal flow pattern in the left side of the heart. The valve should have a large orifice, unrestrictive to a central free flow. It should operate at a low opening pressure without gradients across the valve, and be compatible with high outputs at exercise. The valve should exhibit rapid opening and closure throughout its entire range of pressures without regurgitant flow and without obstruction to the left ventricular output flow. The ideal valve substitute should be attached to the papillary muscles in such a manner as to maintain the valvular-papillary muscle continuity with a minimum of stress to thereby preserve the mechanics and contractural movement of the left ventricle. The valve should provide a uniform distribution of forces and stresses and avoid compressive, tensile or flexure stress during operation. The ideal valve should be constructed entirely of flexible tissue, without mechanical stents and the like. It should exhibit a long life, be durable, resistant to wear and resistant to degeneration, calcification and infection. It should provide normal heart sounds, without noise. It should produce no thrombo-embolic complications, and avoid trauma to blood elements. It should function normally as the left ventricle changes in size. The ideal valve should be easy and reliable to produce and implant.
Mitral valve replacements have not been altogether successful in the past because they have not fully taken into account all of the structural and functional characteristics of the normal mitral valve, including the dynamically changing structure of the mitral ring between systole and diastole, the large inflow orifice, excess leaflet tissue for closure, wall continuity between the mitral ring, papillary muscles and left ventricle, and the other factors mentioned above. Mechanical and bioprosthetic valves have not been altogether successful, because such valves do not have an adequately long life and do not fully simulate the action of a natural valve due to the rigidity of the structure and lack of support to the papillary muscles. Rigid mitral rings and supports do not simulate the physiologic sphincter-like contraction of the natural mitral ring during systole. Patients receiving mechanical valves require anti-coagulant therapy and risk the occurrence of thromboembolic phenomena. Hence, re-operation is necessary in many cases employing mechanical and bioprosthetic valves.
To overcome the problems of mechanical and bioprosthetic valves, many attempts have been made to construct a bicuspid mitral valve formed of entirely an unsupported tissue. However, most of the earlier unsupported valves did not fully account for the factors mentioned above.